Conventionally, stepping motors are widely used as the driving sources for sheet conveyance in printing devices. In a printing device that uses a stepping motor for sheet conveyance, speed fluctuations of the stepping motor serve as one factor of the print density unevenness.
In the stepping motor, an output torque inevitably varies between excitation phases. Due to these variations, the stepping motor involves inherent speed fluctuations corresponding to the frequency component of excitation-phase one cycle. Although the speed fluctuations are small, along with a recent increase in printing speed, the small speed fluctuations in sheet conveyance have come to appear noticeably as print unevenness.
FIG. 11 is a graph illustrating a print unevenness level corresponding to excitation-phase one cycle of the stepping motor.
Regarding FIG. 11, a sublimation type thermal transfer printer is used which employs, as a sheet convey driving source, a 2-phase stepping motor whose torque varies. This sublimation type thermal transfer printer prints an image by driving the stepping motor at a drive pulse frequency of 1440 Hz with a 1-2 phase excitation scheme where 8 steps constitute excitation-phase one cycle. The printing result is read by a scanner, and the read data in the sheet convey direction is Fourier-analyzed, thereby obtaining the graph in which the frequency is plotted along the axis of abscissa and the amplitude spectrum representing the print unevenness level is plotted along the axis of ordinate. As illustrated in FIG. 11, the peak level of the print unevenness appears at 180 Hz which indicates excitation-phase one cycle.
In order to decrease such print unevenness, for example, a motor individual whose output torque does not vary largely is chosen as a stepping motor to be used in a printing device, so that print unevenness resulting from variations in output torque of the stepping motor is suppressed as much as possible. To choose a motor individual, however, increases the cost.
In view of above, as a method of decreasing small speed fluctuations in sheet conveyance, a scheme is known according to which a damper or fly wheel is mounted on the motor shaft or load shaft so that rotation becomes smooth. Another method is also available (see Patent Literature 1). According to this method, a sine-wave current applied in the excitation phase is varied for each excitation-phase one cycle repeatedly to relate to the speed fluctuations being a frequency component in excitation-phase one cycle, among the speed fluctuation inherent to the stepping motor, thereby decreasing the speed fluctuations of the stepping motor.